1. Field of the Invention
This invention relates to metal-layer-containing glazing films. More particularly it relates to improvements in heat-reflecting electrically conductive, substantially transparent metal-layer films for incorporation into glazing structures and other applications and to their preparation.
2. Background Information
A range of metal-layer-containing, light-transmitting structures have been proposed. These structures typically include a transparent support carrying one or more metal layers which are thin enough (10-1000 .ANG., for example) to permit transmission of a substantial fraction of the light shown upon them. These materials present a range of additional properties which have led to commercial applications. For example, the transparent metal layers tend to preferentially reject (reflect and/or absorb) longer wavelength radiation. This has led to glazing materials for architectural and automotive applications which preferentially pass visible light and reject near infrared radiation.
The metal layers in these materials are also electrically conductive. This property has given rise to glazing structures with in situ electrical resistance heating, to glazing structures capable of providing EMF shielding and transparent materials which can be incorporated into electrical and electronic devices such as transparent antistatic layers, transparent membrane switches, photoconductive devices, electroluminescent structures and photosensitive materials and the like.
In some embodiments these metal films are accompanied by dielectric layers which alter the metal layer's optics such as to increase transmissivity, decrease reflectivity at certain wavelengths and the like.
The art-taught structures have a transparent substrate--usually either glass sheet or plastic sheet or film. The substrate carries the metal layer, or in some cases multiple metal layers, and the optional dielectric layers. In some applications, these structures are used as is. In other applications, they are incorporated into more complex glazing systems by lamination or suspension.
Silver is a preferred metal in these structures because of its optical properties, good conductivity and relatively moderate price as compared to noble metals such as gold or platinum. One shortcoming of silver is its substantial chemical reactivity, particularly in the presence of environmental chlorine or sulfur. This reactivity leads to environmental oxidation (corrosion) which manifests itself as decreases in conductivity and transmissivity and as breakdowns in the structural integrity of the bonds between the various layers in the overall glazing material.
One approach to improving silver's stability used heretofore has been to apply transparent inorganic or organic hardcoat overlayers, such as silicon oxide or aluminum oxide overlayers, acrylate or polyolefin overlayers, or the like. Another approach involves complete mixing or alloying (on an atomic scale) of silver with gold or with other inert metals. U.S. Pat. No. 4,234,654 is an example of this approach. While alloying silver offers advantages, it has limitations. For one, when the alloyed silver layer is applied by sputter depositing or another vacuum-deposit method, only a set ratio of silver to the alloying metal, which is dictated by the appropriate composition of the metal source, can be achieved. To alter the ratio requires constructing a new source and reequipping the sputtering or other vacuum-depositing machine with it. For another, although the atomic mixing approach affords a good measure of corrosion resistance, this stability is obtained at the expense of increased visible absorption and increased electrical resistance. This is due to the interruption of the silver lattice structure by "foreign" gold atoms. The chemical stabilization scheme we have discovered achieves chemical inertness with smaller increases in visible absorption and sheet resistivity.
Another patent addressing silver's instability is U.S. Pat. No. 4,565,719 to Optical Coating Laboratories, Inc. which teaches "flash coating" the silver with palladium. While some degree of corrosion protection is afforded by this approach, the inherent absorption and blue coloration of pallidium makes it unacceptable in applications where high visible transparency is desired.